The invention is in the field of high frequency circuits for operating two or more fluorescent lamps connected in electrical series, and more particularly is directed to circuit means for improving starting of the lamps.
High frequency fluorescent lamp circuits generally operate the lamps at a frequency of about 20 kHz or higher, and have the advantage of achieving higher electrical efficiency as compared to the more conventional 60 Hz operation. In both low-frequency (60 Hz) and high-frequency circuits, lamp starting is improved by providing a conductive strip or coating along the bulb, or by positioning the bulbs adjacent and parallel to an electrically conductive reflector or fixture for holding the bulbs, a spacing of about one-half inch between the bulbs and the conductive reflector or fixture being suitable for aiding lamp starting. The improved starting is in the form of faster and more reliable starting, and also satisfactory starting at a lower circuit voltage applied across the lamps.
In a two-lamp circuit having two fluorescent lamps connected in electrical series, it is conventional to connect a starting capacitor across a first one of the lamps, and it functions by applying substantially all of the starting voltage across the second lamp, causing its electrical discharge to start, whereupon the lowered voltage drop of the second lamp applies sufficient voltage across the first lamp for starting it; the two lamps appear to start simultaneously. U.S. Pat. No. 4,237,403 to Davis, incorporated herein by reference thereto, shows, in FIG. 3, a high-frequency circuit for starting and operating two fluorescent lamps connected in electrical series, the circuit having a starting capacitor 94 connected across one of the lamps 70 and functioning to aid starting of the other lamp 71, as described in column 8, lines 49-54. The circuit also has a capacitor 56 connected across both lamps, and it resonates with the transformer secondary winding 55 to aid lamp starting, as described in column 5, lines 61-67.
While the above-described techniques are useful for achieving improved reliability of lamp starting, and achieve circuit economy by insuring lamp starting at lower circuit voltage, the desirability exists for further means of improving lamp starting in high-frequency lamp systems.